In recent years, a light-emitting device and a display that use a light-emitting element formed by using an organic material have been actively developed. Since the light-emitting, which is manufactured by interposing an organic compound layer between a pair of electrodes, itself emits light unlike a liquid crystal display device, no light source such as a backlight is necessary, and the element itself is quite thin. Therefore, the light-emitting element is advantageous when a thin and lightweight display is manufactured.
The emission mechanism of the light-emitting element is said to be as follows: an electron injected from a cathode and a hole injected from an anode are recombined in the luminescence center in an organic compound to form a molecular exciton; and energy is released to emit light when the molecular exciton returns to the ground state. As the excited state, a singlet excited state and a triplet excited state are known, and it is believed that light can be emitted through either excited state.
The organic compound layer sandwiched between the electrodes often has a laminated structure, and a function-separation laminated structure of “a hole transporting layer, a light-emitting layer, and an electron transporting layer” is typical as this laminated structure. When a layer formed by using a highly hole transporting material and a layer formed by using a highly electron transporting material are disposed respectively on the side of an electrode that functions as an anode and on the side of an electrode that functions as a cathode to interpose a light-emitting layer in which a hole and an electron are recombined therebetween, holes and electrons can be transported efficiently, and further the recombination probability of holes and electrons can also be increased. This structure provides a quite high luminous efficiency, and is thus employed for most of light-emitting display devices currently under development (for example, Non-Patent Document 1).
[Non-Patent Reference 1]
    Chihaya Adachi et al., Japanese Journal of Applied Physics, Vol. 27, No. 2, pp. L269-L271 (1988)
In addition, as other structures, there are a structure of stacking in the order of a hole injecting layer, a hole transporting layer, a light-emitting layer, and an electron transporting layer from an electrode that functions as an anode, and a structure of stacking in the order of a hole injecting layer, a hole transporting layer, a light-emitting layer, an electron transporting layer, and an electron injecting layer from an electrode that functions as an anode, and the respective layers are formed by using materials that specialize in the respective functions. It is to be noted a layer that has two or more of these functions, for example, a layer that has both functions of a light-emitting layer and an electron transporting layer, may be used.
The organic compound layer typically has a laminated structure as describes above. However, a layer formed to have a single layer structure or a mixed layer may be used, and a light-emitting layer may be doped with a fluorescent dye or the like.
However, this light-emitting element has problems with heat resistance and durability. Since this light-emitting is formed by stacking organic thin films using organic compounds as described above, fragility of the thin films using the organic compounds is considered to be a cause of the problems.
On the other hand, there is also an example of manufacturing a light-emitting element by applying not an organic thin film but a layer in which organic compounds (a hole transporting compound, an electron transporting compound, and a luminescent compound) are dispersed in a skeleton formed by a siloxane bond (for example, Patent Document 1 and Non-Patent Document 2). Further, in Patent Document 1, it is also reported that the durability and heat resistance of the element are improved.
[Patent Reference 1]
    Japanese Patent Application Laid-Open No. 2000-306669[Non-Patent Reference 2]    Tony Dantas de Morais et al., Advanced Materials, Vol. 11, No. 2, pp. 107-112 (1999)
However, in the light-emitting elements disclosed in Patent Document 1 and Non-Patent Document 2, current is hard to flow as compared with conventional light-emitting elements since the organic compounds are dispersed in the insulating skeleton formed by the siloxane bond.
In these light-emitting elements, since the luminance gets higher in proportion to an applied current, the fact that current is hard to flow means that the voltage for obtaining a predetermined luminance (driving voltage) also gets higher. Thus, the power consumption is increased.
In addition, in order to suppress short circuit of a light-emitting element due to dust and the like, it is effective to make the film thickness of the light-emitting element thicker. However, when the film thickness is made thicker in the light-emitting element that has the structure as shown in Patent Document 1 or Non-Patent Document 2, the increase in driving voltage is further exposed.